When comparing Emacs vs Spacemacs, the Slant community recommends Spacemacs for most people. In the question“What are the best programming text editors?” Spacemacs is ranked 5th while Emacs is ranked 10th. The most important reason people chose Spacemacs is:
Spacemacs combines the Emacs platform (with the full power of the Emacs plugin ecosystem) and the Vi keybindings (via EViL), all in the same box.
Specs
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Pros
Pro Keyboard-focused, mouse-free editing
Emacs can be controlled entirely with the keyboard. While true, I often find the mouse and menus handy for those lesser-used commands. An aide-memoir.
Pro Total customizability
Customizations can be made to a wide range of Emacs' functions through a Lisp dialect (Emacs Lisp). A robust list of existing Lisp extensions include the practical (git integration, syntax highlighting, etc) to the utilitarian (calculators, calendars) to the sublime (chess, Eliza).
Pro It's also an IDE
You can debug, compile, manage files, integrate with version control systems, etc. All through the various plugins that can be installed.
Pro Works in terminal or as a GUI application
You can use Emacs' command line interface or graphical user interface.
Pro Self documenting
Emacs has extensive help support built-in as well as a tutorial accessed with C-h t.
Pro Great documentation
With 30+ years of use the Emacs documentation is very thorough. There are also a lot of tutorials and guides written by third parties.
Pro Free
Licensed under GNU GPL.
Pro Mini buffer
You can pass complicated arguments in the mini buffer.
Pro Ubiquity
Fully compliant GNU-emacs is available on many platforms, and they all understand .emacs configuration files.

Pro Vi keybindings through Evil mode
Evil mode emulates vim behaviors within Emacs. It enables Vi users to move inside the Emacs universe.
Pro Rectangular cut and paste
Emacs can select rectangularly.
Pro Lisp customizations
With lisp customization, any behavior of Emacs can be changed. Update with pre-release patch can be also applied without recompiling the whole Emacs.
Pro Visual selection and text objects with Evil
Evil is an extensible vi layer for Emacs. It provides Vim features like Visual selection and text objects.
Pro dabbrev-expand (Alt-/)
Dynamic word completion.
Pro Provides org-mode
Advanced planning and publication which can start as a simple list.
Pro Integrates planning in your development process
You can jump straight from your org-mode files to programming tasks - and back - and build a seamless workflow.
Pro Support multi-line editing, multiple frame, powerful paren, crazy jumping style
Review the "Emacs Rocks" video.
Pro Enormous range of functionalities (way beyond simple "text editing")
Through its programmability, a very broad range of functionalities can be integrated in emacs, turning it even into a "single point of contact" with the underlying operating system.
Pro Helm plugin adds even more power to Emacs
Powerful commands, search, and more with the Helm plugin.
Pro Has been widely used for a long time
Pro Cross-platform
Works on Linux, Windows, Macintosh, BSD, and others.
Pro GTK+ widgets support
Since version 25 you can run GTK widgets inside Emacs buffers. One of these is the WebKitGTK+, which allows the user to run a full-featured web browser inside Emacs with JavaScript and CSS support among other things.
Pro Versatile
Emacs is great for everything.
Pro Gnus
Managing several large mailing lists has never been easier using Gnus. The threading commands and the various ways of scoring articles means that I never miss important messages/authors, etc. A joy to use.
Pro eshell is cross platform
Pro Interactive Shells
Emacs has a number of shell variants: ansi-term, shell, and eshell.
Pro Combines the best parts of Vi and Emacs
Spacemacs combines the Emacs platform (with the full power of the Emacs plugin ecosystem) and the Vi keybindings (via EViL), all in the same box.
Pro Simple but powerful configuration architecture
At the heart of Spacemacs, the configuration layers group packages configuration into semantic units that can be toggled on and off. The architecture is simple but powerful, allowing the user to easily manage configuration dependencies between hundreds of packages.
Pro Community-driven configuration
Spacemacs is the biggest community-driven Emacs starter-kit.
Pro Above average documentation quality
Documentation is mandatory for each new configuration layer and can be accessed directly within the editor in Org format.
Pro Cross-platform
Emacs runs on Gnu/Linux, Mac OS X, and Windows.
Pro Completely configured out of the box
Stuff like version control, file management, good default theme are all configured out of the box.
Pro Great support from the community
The community surrounding Spacemacs is very active and there is a welcoming gitter chat for users to ask questions.
Pro Mnemonic and consistent keybindings
Space-lead key bindings are organized in mnemonic namespaces. For instance, buffer actions are under SPC b
, file actions are under SPC f
, project actions are under SPC p
, search actions are under SPC s
, and so on. Keybindings are consistent across the whole distribution thanks to a set of conventions.
Pro Fast-paced development
New functionalities and fixes are added to Spacemacs every day, while release cycles are short.
Pro Can be controlled fully with the keyboard
There's no need to reach for the mouse again since Spacemacs can be fully controlled with keyboard.
Pro Lowers the risk of RSI by using the spacebar as leader
Spacemacs got its name from the fact that it is uses the space bar as a default leader key. The key was chosen because it is easy to press and to hopefully lower the risk of RSI.
Pro Gradual learning curve
Evil package is a first class citizen and Spacemacs embraces it from day one. Evil package allows Vim users to be productive very quickly while still allowing regular Emacs users to use Spacemacs.
Pro Remote file editing
Files can be edited in Spacemacs remotely.
Pro Great note-taking and agenda "app" built-in
Allows for great organization applications that can be saved in future-proof format, plain text, can be integrated with LaTeX, markdown, HTML, Literate Programming and be committed to source control.
Pro Manage many code bases easily
Pro Offers a number of practical features
Spacemacs has some great features for taking notes, tracking to-do lists, and tracking time.
Pro Works well with Common Lisp
Pro LaTeX support
LaTeX allows for auto-completion, syncing, and more.
Pro Can work in terminal mode
Sometimes you only have terminal access, over ssh or something.
Pro Daemon support
Has great daemon support, which can mitigate the issue of slow startup.
Pro Manage R files easily
Pro Easily extended with community plugins
Pro Great CFEngine support
Syntax highlighting and org-babel extensions.
Pro Great Clojure support
Cons
Con Learning curve is long
While it's better than it used to be, with most functions being possible through the menu, Emacs is still quite a bit different from your standard editor. You'll need to learn new keyboard shortcuts.
Con Sometimes the extensibility can distract you from your actual work
If I ever want to lose half a day, I'll start by tweaking my .spacemacs config file.

Con Chorded keyboard combinations can be baffling
For example, for navigation it uses the b, n, p, l keys. Which for some people may seem strange. But don't worry, you can change it.
Con Hard customization
For customization, you need to learn Lisp
Con A lot of jokes in this serious software
Con Using Emacs on a new machine without your .emacs file
Con Can be quite glitchy at times
Spacemacs combines many packages from many different authors that were never designed to work together. Sometimes they interact in unexpected ways, and things randomly break as one package interferes with another's features. This combined with delayed auto-loading of packages in unpredictable order, different modes for different file types, frequent package updates, and necessary customization by selection of layers and packages, can make these glitches hard to reproduce. It takes a lot of emacs know-how to fix these problems. Fortunately there is a very active community willing to help with these problems, but it might take a while.

Con Functionality layers of complicated configuration
To configure Spacemacs, settings for Emacs/Evil/Spacemacs may need editing. It's not always clear which need to be changed or how to change settings globally: sometimes hooks are needed, other times Spacemacs provides options.

Con Slow startup time
Although configuration is heavily loaded, the starting time of Spacemacs is usually between two and five seconds. Emacs can be run as a daemon though which reduces the client's startup time to a few milliseconds.
Con High CPU and unresponsive at times
There are occasions when Spacemacs would suddenly consume a LOT of CPU and then other times would become completely unresponsive. This instability took place only 6 months or so ago. Restarting Spacemacs can fix it for a while but perhaps this issue is already fixed in newer versions.
Con Complex learning difficulty
You must be familiar with either Vim or Emacs. In addition, you should be familiar with the unique features of Spacemacs. The Layer concept of replacing Emacs settings is still difficult and abstract compared to modern editors.